1. Field of the Invention
The present invention relates to an apparatus for and a method of separating coarse particles from a stream of gas and a mixture of coarse and fine particles discharged from a vertical mill. More particularly, this invention relates to a dynamic particle classifier for aerodynamic separation of coarse particles from a stream of gas and particles discharged from a vertical mill, and a separating method for a dynamic particle classifier.
2. Description of the Related Art
A dynamic particle classifier generally includes a rotating wheel of vanes. The purpose of such a particle classifier is to remove oversized particles from an original stream (for example, a stream of gas) without removing too many fine particles. The sharpness of the particle size distribution in the outlet stream of a dynamic classifier is usually increased by combining a set of static vanes and a set of rotating vanes in the classifier.
A particle classifier is often used in combination with a vertical mill, such as a coal mill or a cement mill, which grinds coarse raw material to small particles. In this arrangement, a stream of gas and particles is discharged from the mill to the classifier, which separates the coarsest fraction of the particles from the stream and returns the separated particles to the mill for re-grinding.
In a conventional dynamic classifier, such as disclosed, e.g., in U.S. Pat. No. 4,919,341, a stream of gas and particles discharged from a coal mill is first imparted in rotation by static vanes and then directed from outside to a rotating wheel of vanes. Gas and fine particles pass through the rotating vanes to a central volume, and flow upwards to exit the classifier through an outlet opening. Larger particles attempting to move through the rotating vanes will be thrown back and drop through a central outlet opening of the classifier back to a mill.
A disadvantage of the conventional dynamic classifier design is that an incoming stream enters the rotating wheel radially inwardly and a flow of separated coarse particles is directed from the wheel outward at the same time. These opposing flows lead to a collision of the streams, resulting in an increased resistance to the flow of gas. The flow of gas through the classifier must overcome the resistance of the opposing flow, resulting in an increased pressure loss in the classifier.
In a classifier combined with a vertical mill, the separated coarse particles are directed, together with fresh raw material, to the center of the mill. The material is usually ground in the mill by rollers on a rotating plate, while the material moves from the center of the plate toward its peripheral edge. At the edge of the rotating plate, the ground material is conveyed up to the classifier, as explained above, by a stream of gas.
The radially extending vanes of the rotating wheel usually form a vertical cylinder or a downward tapering cone. U.S. Pat. No. 5,251,831 discloses a dynamic classifier, in which the rotating wheel forms a downward tapering cone, and each rotating vane is downwardly inclined, i.e., designed such that its lower portion is trailing with respect to its upper portion. Thereby, the rotating wheel does not change the direction of the separated particles completely opposite to the incoming flow, but partly downwards, toward the central outlet opening of the inner cone. Thus, the flow of separated particles is not opposing, but merely traversing, the incoming flow. This reduces the pressure loss to some extent compared to that in a conventional dynamic classifier.
British patent publication GB 1,280,062 discloses a classifier in which the centrifugal classification by static vanes is aided by providing a vertically-oriented axial fan at the upper portion of the classification space. A flow of coarse particles, separated by the axial fan, crosses the stream of gas and particles flowing through the static vanes, and tends to be mixed with it.
All of the above-described classifier designs involve interference between different streams, and thus, tend to suffer from increased pressure losses and reduced classifying efficiency.